Image intensifier tube having a multi-radius photocathode

ABSTRACT

An image intensifier tube including a multi-radius photocathode having less curvature adjacent its periphery than in the central portion thereof.

Umted States Patent 1151 3,697,795

Braun et al. 1 Oct. 10, 1972 [54] IMAGE INTENSIFIER TUBE HAVING A [56] References Cited MULTI-RADIUS PHOTOCATHODE UNITED STATES PATENTS [721 "venwrs: Marti" Bram, 2,179,083 11/1939 Bruche et a] ..313/102 x Stanley 5mg", 2,824,986 2/1958 Rome .250/213 UT x bmh Of New York 2,743,195 4/1956 Longini ..250/213 VT x [73] Assignee: The Machlett Laboratories, lncor- 3,417,242 12/1968 Windebank 50/2l3 VT X -m springdale, Conn 2,520,190 8/1950 Amdursky ..3l3/65 R X 2,974,244 3/1961 Niklas ..250/2l3 VT X [22] F1led: Nov. 20, 1970 I21 App]. 9 31 Primary Examiner-Stanley D. Mlller, Jr.

Att0rneyHarold A. Murphy and Joseph D. Pannone [52] US. Cl. ..3l3/l02, 250/213 VT 57 S CT 51 ln1.Cl ..H01j 39/00, HOlj 31/50 [58] Field of Search 313/65 R, 94, 102; 250/213 VT An lmage mtenslfier tube lnciudlng a mult1-rad1us photocathode having less curvature adjacent its periphery than in the central portion thereof.

9 Claims, 5 Drawing Figures PATENTEDum 10 I972 3.697, 795

sum 1 or 2 ffy F/az IMAGE INTENSIFIER TUBE HAVING A MULTI- RADIUS PHOTOCATHODE BACKGROUND OF THE INVENTION This invention is related generally to image converter tubes and is concerned more particularly with an image intensifier tube having an aspherical photocathode for producing a substantially flat visible image having uniform resolution and minimum edge distortion.

An -image converter tube generally comprises a photocathode which, in response to an incident radiant image, produces a corresponding electron image, and a spaced, coaxially disposed anode assembly including an imaging screen which, in response to an impinging electron image, produces a visible light image. Usually, the anode assembly is maintained at a high positive potential with respect to the photocathode in order to establish therebetween a strong electrostatic field which will amplify the electron image by accelerating it toward the imaging screen.

One type of image intensifier tube has a spherically curved photocathode which emits a converging electron image and an anode assembly having an axially extending portion which is provided with an end aperture through which the electron image must pass in order to impinge on an axially aligned imaging screen. Thus, an image intensifier tube of the described type has an axis of symmetry which extends from the center of the spherically curved photocathode, through the center of the anode aperture and terminates in the center of the axially aligned imaging screen. Usually the apertured end portion of the anode assembly has a smaller diameter than the photocathode and an external contour which, in conjunction with the opposing surface of the photocathode, aids in establishing therebetween a radially symmetric electrostatic field having spherically curved, equipotential surfaces. This electrostatic field provides optimum focusing conditions for directing and converging the electron image emitted by the photocathode toward a crossover region centered around the axis of symmetry and adjacent the anode aperture. Generally, one or more tubular grid electrodes are longitudinally disposed between the photocathode and the anode assembly for the purpose of adjusting the shape of the electrostatic field and directing the electron image toward the imaging screen.

After the electron image passes through the crossover region, it is inverted and enlarges as it travels toward the imaging screen. However, the enlarging, inverted image has a spherical curvature which is opposite to the curvature of the photocathode. Consequently, when the center of the electron image contacts the imaging screen, the edge portions of the image are curved away from the imaging screen. As a result, the edges of the visible image produced by this electron image will be bowed radially inward toward the center of the image.

The problem of achieving a controlled electron image for the purpose of minimizing distortion in the resulting visible image has been investigated by others but without significant success. For example, U. S. Pat. No. 2,974,244 granted to W. F. Niklas and issued on Mar. 6, 1961 discusses this problem and teaches that it can be corrected by providing the described image intensifier tube with a photocathode having an essentially ellipsoidal configuration. Thus, the Niklas photocathode has a surface curvature which increases progressively outwardly from the center thereof. However, the more curved edge portions of the Niklas photocathode will result in electrons emitted therefrom being focused more sharply than the electrons emitted from peripheral portions of a conventional, spherically curved photocathode. Consequently, the image intensifier tube disclosed in the Niklas patent will produce an electron image surface having correspondingly greater edge curvature and a resulting visible image having greater distortion than a conventional image intensifier tube. Thus, the prior art does not provide an adequate solution to the problem of minimizing edge distortion in visible images produced by image intensifier tubes of the described type.

SUMMARY OF THE INVENTION Accordingly, this invention provides an image intensifier tube having a multi-radius photocathode which produces a visible image having substantially uniform resolution on the imaging screen of the tube. An image intensifier tube constructed in accordance with this invention comprises a generally tubular envelope closed at one end, within which end is an outwardly curved photocathode having less curvature adjacent the periphery thereof than in the central portion. The photocathode ma be supported on the inner surface of the input faceplate and supported in axial spaced relatio nship with a frusto-conical anode sleeve which is longitudinally disposed on the axial centerline of the tube. The anode sleeve has an apertured, smaller diameter end disposed in opposing relationship with the central portion of the photocathode and an open, larger diameter end disposed adjacent a flat imaging screen. The screen is deposited on one surface of a transparent plate which may close the other end of the tubular envelope. One or more tubular grid electrodes are longitudinally disposed between the photocathode and the anode to aid in focusing an electron image emitted by the photocathode on the imaging screen.

BRIEF DESCRIPTION OF THE DRAWING For a more complete understanding of this invention, reference is made to the accompanying drawing wherein:

FIG. 1 is an axial sectional view of an image intensifier tube embodying the photocathode of this invention;

FIG. 2 is a diagrammatic representation of a typical electrostatic field obtainable with the photocathode of this invention;

FIG. 3 is a diagrammatic representation of an alternative embodiment of this invention;

FIG. 4 is a diagrammatic representation of another alternative embodiment of this invention; and

FIG. 5 is a diagrammatic representation of another alternative embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly to the drawing, there is shown in FIG. 1 an image intensifier tube 10 comprising a generally tubular envelope 12 having a large diameter portion 13 which terminates at one end of the envelope in an outwardly extending flange l4. Peripherally sealed to the flanged end of envelope 12 is a transversely disposed, aspherical faceplate 16 having a convex outer surface and a concave inner surface. The faceplate 16 serves as the input faceplate of tube and comprises a thin sheet of material which is transparent to a selected band of electromagnetic wavelengths. For purposes of illustrating the novel features of this invention, it will be assumed that tube 10 is an X-ray image intensifier tube and, therefore, input faceplate 16 is made of a material, such as glass or aluminum, for example, which is transparent to incident X-ray energy. However, it is to be understood that this invention is equally applicable to image intensifier tubes having input faceplates which are transparent to wavelengths in other portions of the electromagnetic spectrum.

Disposed on the inner surface of faceplate 16 is a conforming layer 18 of florescent material, such as silver activated zinc sulfide, for example, which is sensitive to incident X-ray energy. The inner surface of layer 18 is coated with a conforming film 20 of compatible material, such as aluminum oxide, for example, which is transparent to visible light energy. Deposited on the inner surface of film 20 is a conforming layer 22 of photoemissive material, such as cesium antimonide, for example, which functions as the photocathode of the tube. Thus, the fluorescent layer 18, compatible film 20 and the photocathode 22 each have an aspherical configuration which conforms to the supporting input faceplate 16.

In operation, incident X-ray energy conveying a radiant image of an external object passes through the transparent faceplate 16 and impinges on the fluorescent layer 18. Accordingly, the layer of fluorescent material emits photons of visible light and thereby produces a corresponding visible light image of the external object. The photons of light energy pass through the transparent film 20 and impinge on the photocathode 22. As a result, the photocathode emits electrons from its concave inner surface in accordance with the impinging photons thereby producing a corresponding electron image of the external object. Thus, the aspherically curved layers 18, 20 and 22, respectively, comprise an input screen assembly which receives a radiant image of an external object and converts it into a corresponding electron image which may be amplified and converted into a bright visible image.

In instances where the spectral frequency and intensity of the incident radiant energy is sufficiently high to produce an electron image by impinging directly on the photocathode 22, the intermediate fluorescent layer 18 and film 20 are not required. Consequently, the resulting intensifier tube may have an input faceplate which is made of lead-free glass, for example; and the photocathode 22 may be deposited directly on the inner surface of the input faceplate 16. On the other hand, some types of image intensifier tubes do not have the photocathode 22 supported on the inner surface of the faceplate 16, but have it disposed on the inner surface of a glass or metallic dish which is axially spaced from the input faceplate. In these image intensifier tubes, the photocathode need not conform to the configuration of the input faceplate. in any case, however it is essential to the practicing of this invention that the ,electron emitting surface of the photocathode be merge with a small diameter portion 17 which terminates at the other end of envelope l2. Peripherally sealed to the small diameter end of envelope 12 is a transversely disposed, flat faceplate 26 which serves as the output faceplate of the tube. The faceplate 26 may be made of any suitable material which is transparent to visible light, such as glass, for example.

Within the envelope 12, a flat plate 28 of transparent material, such as glass, for example, is positioned in substantially parallel adjacent relationship with the output faceplate 26. The plate 28 is disposed coaxially with the axial centerline of tube 10 and supports on its inner surface an imaging screen 30 comprising a layer of fluorescent material, such as zinc cadmium sulfide, for example, which is sensitive to impinging electrons. The inner surface of imaging screen 30 is coated with a thin film 31 of light reflecting material, such as aluminum, for example, which is transparent to high energy electrons. The supporting plate 28 is peripherally attached to a large diameter end of an axially extending, frusto-conical sleeve 32. The opposing smaller diameter end of sleeve 32 is disposed in spaced, opposing relationship with the concave inner surface of photocathode 22 and is provided with a centrally disposed aperture 36 through which the electron image emitted by photocathode 22 must pass in order to im pinge on the imaging screen 30. Thus, an imaginary axis of symmetry 35 extends from the center of the concave inner surface of photocathode 22, through the center of aperture 36 and terminates in the center of imaging screen 30.

In operation, a high positive potential relative to the photocathode potential is applied to the sleeve 32. Consequently, there is established between the photocathode 22 and the anode sleeve 32 a strong electrostatic field having lines of force which terminate at one end in the concave inner surface of the photocathode and at the other end in the frusto-conical, outer surface of sleeve 32. Since the diameter of the photocathode 22 is much larger than the small diameter end of sleeve 32, such as ten times larger, for example, the electrostatic field lines of force will converge from points on the larger diameter photocathode 22 to corresponding points on the smaller diameter end of sleeve 32. The associated equipotential surfaces, which are perpendicular to the lines of force, will follow the concave curvature of the photocathode, in the vicinity of the photocathode, and will follow the frustoconical contour of sleeve 32 in the vicinity of aperture 34. Thus, if the photocathode is curved spherically, as in conventional image intensifer tubes of the described type, the equipotential surfaces of the resulting electrostatic field in the vicinity of the photocathode will be curved spherically also. Therefore, the associated field lines of electric force will extend radially from points on the surface of the photocathode to a crossover region centered around the axis of symmetry. However, electrons emitted from the central portion of the spherically curved photocathode will cross the axis of symmetry and come to a focus at a greater distance from the photocathode than electrons which are emitted from annular portions adjacent the periphery of the photocathode. As a result, after passing through the crossover region, the electron image has a spherical curvature which is opposite to the curvature of the photocathode and will produce a distorted visible image.

In order to avoid the described distortion defects, the image intensifier tube of this invention is provided with a photocathode which is curved more sharply in the central portion thereof than in annular portions adjacent its periphery, as shown in FIG. 2. Thus, when viewed from the imaging screen 30 the photocathode of this invention is an asperically curved, multi-radius photocathode including an inner photoemissive surface having a sharply concave central portion and surrounding annular portions which are less curved than the central portion. Consequently, in the vicinity of the photocathode, the equipotential surfaces 37 of the resulting electrostatic field have respective central regions which are curved more sharply than their respective outer annular regions. Accordingly, the associated electric field, initially, exerts a stronger converging influence on the electrons emitted from the central portion of the photocathode. As a result, electrons emitted from the marginal portions of the photocathode will cross the axis of symmetry at a greater distance from the photocathode than would be the case in conventional image intensifier tubes. Consequently, the electrons emitted from the marginal portions will come to a focus in substantially the same plane as the electrons emitted from the central portion of the photocathode. Furthermore, the less curved annular portions adjacent the periphery of this novel photocathode will introduce less distortion in the resulting image than the more curved annular portions adjacent the peripheries of prior art photocathodes. Thus, after passing through the crossover region, the electron image will produce on the imaging screen 30 a substantially flat visible image having uniform resolution and minimum distortion.

Since this invention requires that the photocathode be sharply curved in the central portion, the surrounding annular portions of the photocathode may extend radially away from the axis of symmetry at a constant angle therewith, as shown in FIG. 3. Thus, the photoemissive surface of this photocathode may have a configuration corresponding to a hyperbolic surface of revolution, which is obtained by rotating an axially extending hyperbola about the axis of symmetry. Consequently, the equipotential surfaces 37a of the electrostatic field in the vicinity of the photocathode 22a have generally conforming hyperbolic curvatures. On the other hand, the marginal annular portions of this photocathode may extend radially away from the axis of symmetry at substantially a right angle therewith, as shown in FIG. 4. Thus, the marginal annular portions of this photocathode 22b have respective radii of curvatures which are substantially infinite in magnitude as compared to the radius of curvature of the central portion of the photocathode. Accordingly, the conforming equipotential surfaces 37b of the resulting electrostatic field in the vicinity of this portion of the photocathode 22b are very nearly perpendicular to the axis of symmetry also. Alternatively, the marginal annular portions of the photoemissive surface may have a reverse curvature as compared to the curvature of the central portion, as shown in FIG. 5, thus having respective minus radii of curvatures with respect to the radius of curvature of the central portion. As a result, the conforming marginal portions of the equipotential surfaces 370 in the vicinity of the photocathode 22c will be curved opposite to, but less sharply than, the similarly conforming central portions thereof. Also, when viewed from the imaging screen side of the photocathode, the photoemissive surface thereof may have an intermediate annular portion which is convex, but less sharply curved than the central concave portion. Thus, the conforming equipotential surfaces 370 in the vicinity of the photocathode 22c will be curved accordingly.

The successively contiguous, annular portions of the photocathode which surround the more sharply curved central portion thereof need not have respective radii of curvatures which increase monotonically with increasing radial distance of the associated annular portions from the axial center of the photocathode. However, it is essential when practicing this invention, that the marginal annular portions of the photoemissive surface, that is, adjacent the periphery of the photocathode, be less curved than the central portion thereof. In this manner, electrons emitted from these marginal annular portions of the photocathode will not be converged as strongly toward the axis of symmetry by the electrostatic field as the electrons emitted from the more sharply curved central portion of the photocathode will be. Consequently, the electrons emitted from the marginal annular portions of the photocathode will be brought to a focus in substantially the same image plane as the electrons emitted from the central portion of the photocathode.

One or more tubular grid electrodes, such as the axially aligned series of respective electrodes 38, 39 and 40, for example, may be positioned longitudinally between the photocathode 28 and the anode sleeve 32 and coaxially disposed with the axis of symmetry. As shown in FIG. 2, the respective potentials applied to these electrodes may be adjusted to shape the electrostatic field for optimum focusing condition whereby an electron image emitted by the photocathode 22 is directed through the anode aperture 36 and toward the imaging screen 30. Thus, the electron image is amplified by being accelerated to higher kinetic energy levels and impinges on the imaging screen 30 with sufficient energy to produce a bright visible image which is substantially free of edge distortion. The size of this image may be magnified or minified by varying the voltages applied to the grid electrodes 38, 39 and 40, respectively. The zooming" effect of this type of image intensifier tube is fully disclosed and described in U. S. Pat. No. 3,417,242 granted to R. W. W. Windebank which issued on Dec. 17, 1968 and is assigned to the assignee of this invention.

Generally, the electrons emitted by the photocathode leave respective discrete areas of the photoemissive surface and follow curved paths which cross the axis of symmetry at respective distances from the photocathode and terminate at corresponding discrete areas of an inverted image. Thus, in order to have photocathode. The electron paths and associated cros-,

sover distances are determined, in part, by the strength and shape of the electrostatic field. Consequently, one of the several reasons why the shape of the photoemissive surface is important is that it determines the shape of the electrostatic field in the vicinity of the photocathode.

However, it has been found that a slight change in the shape of the photocathode produces only a correspondingly slight change in the shape of the electrostatic field adjacent thereto which, in itself, does not have a substantial effect on changing the paths of electrons emitted therefrom or their associated crossover distances. On the other hand, this slight change in the contour of the photoemissive surface may cause a significant change in the angle at which the electrons emitted therefrom enter the electrostatic field with respect to the axis of symmetry. It has been found that a slight change in this emission angle causes a considerable change in the paths of the emitted electrons and their approach to the axis of symmetry. As a result, the crossover distances for these emitted electrons changes significantly and the electrons come to a focus in a correspondingly different transverse plane. Generally, if the crossover distances of the emitted electrons increases, the electrons will come to a focus at a greater distance from the photocathode than prior to the contour change.

Since an image produced by a conventional, spherically curved photocathode has curved edge portions which are located closer to the photocathode than the central portion, it follows that the image can be flattened by moving the edge portions away from the photocathode and into substantially the same transverse plane as the central portion of the image. This objective may be achieved by focusing the electrons emitted from the marginal portions of the photocathode at a greater distance from the photocathode than the distances at which these electrons would be focused if emitted from marginal portions of a spherically curved photocathode. Con-- sequently, the marginal portions of the photocathode of this invention are less curved than its central portion. As a result, electrons emitted from the marginal portions of this novel photocathode approach the axis of symmetry more gradually and, therefore, cross it at a greater distance from the photocathode as compared to electrons emitted from marginal portions of a conventional, spherically curved photocathode. Thus, the electrons emitted from the less curved marginal portions of this inventive photocathode come to a focus at a greater distance from the photocathode and in substantially the same transverse plane as the electrons emitted from the more curved central portion of the photocathode. In this manner, a substantially flat visible is produced which has less distortion and more uniform resolution qualities than the visible image obtained with a spherically curved photocathode. Surprisingly, it has been found that the resulting image remains in focus even when the described image intensifier tube is operated to zoom or magnify a portion of the image.

Thus, there has been disclosed herein an image intensifier tube having an input screen assembly including an aspherical, multi-radius photocathode having a ,concave inner surface which is curved more sharply in the central portion than in coaxial, encircling, annular portions. The successive, contiguous annular portions of this photocathode have respective radii of curvatures of greater magnitude than the radius of curvature of the more sharply curved central portion. This novel photocathode shapes adjacent equipotential surfaces of an electrostatic field such that an electron image emitted by the photocathode comes to a focus as a substantially flat electron image. Consequently, when this electron image impinges on an output imaging screen, it produces a bright visible image which is substantially free of edge distortion.

From the foregoing, it will be apparent that all of the objectives of this invention have been achieved by the structures shown and described herein. It will be also apparent, however, that various changes may be made by those skilled in the art without departing from the spirit of the invention as expressed in the appended claims. It is to be understood, therefore, that all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A photocathode having an aspherically curved, effective electron emitting surface comprising a concave central portion and an encircling marginal portion adjacent its periphery, the marginal portion having a radius of curvature greater in magnitude than the radius of curvature of the central portion and the entire electron emitting surface including both of said portions being effective in the formation of an electron image in response to an incident radiational image.

2. A photocathode as set forth in claim 1 wherein the marginal portion has a radius of curvature which extends in the same direction as the radius of curvature of the central portion.

3. A photocathode as set forth in claim 1 wherein the marginal portion has a radius of curvature which extends in the opposite direction as compared to the radius of curvature of the central portion.

4. A photocathode as set forth in claim 2 wherein the effective electron emitting surface comprises a hyperbolic surface of revolution.

5. An image intensifier tube comprising:

an evacuated envelope having an input faceplate and an output faceplate;

an aspherically curved photocathode disposed within the envelope adjacent the input faceplate;

said photocathode including an effective electron emitting surface having a concave central portion and an encircling marginal portion adjacent its periphery;

said marginal portion having a radius of curvature greater in magnitude than the radius of curvature of said central portion and the entire electron emitting surface including both of said portions being effective in the formation of an electron image in response to an incident radiational image; and

a substantially flat imaging screen disposed within the envelope adjacent the output faceplate and axially aligned therewith.

6. An image intensifier tube as set forth in claim wherein a layer of fluorescent material is coaxially disposed between the input faceplate and the photocathode.

7 An image intensifier tube as set forth in claim 5 wherein an anode sleeve is coaxially disposed between the photocathode and the imaging screen and is axially spaced from the photocathode.

8. An image intensifier tube as set forth in claim 7 wherein a tubular focusing electrode is coaxially disposed between the photocathode and the anode sleeve and axially spaced therefrom.

9. An image intensifier tube comprising:

a tubular envelope having an input faceplate at one end and an output faceplate at the other end, the input faceplate having an aspherically curved inner surface including a concave central portion, a contiguous intermediate annular portion and a successively contiguous encircling marginal portion adjacent its periphery, the intermediate and marginal portions having respective radii of curvatures greater in magnitude than the radius of curvature of the central portion;

a conforming layer of photoemissive material on the inner surface of the input faceplate, the entire photoemissive layer including the concave central portion, the intermediate annular portion and the encircling marginal portion adjacent its periphery being effective in the formation of an electron image in response to an incident radiational image;

an anode sleeve disposed within the envelope in spaced coaxial relationship with the photoemissive layer; and

a substantially flat imaging screen disposed within the envelope in spaced coaxial relationship with the anode sleeve and located adjacent said output faceplate. 

1. A photocathode having an aspherically curved, effective electron emitting surface comprising a concave central portion and an encircling marginal portion adjacent its periphery, the marginal portion having a radius of curvature greater in magnitude than the radius of curvature of the central portion and the entire electron emitting surface including both of said portions being effective in the formation of an electron image in response to an incident radiational image.
 2. A photocathode as set forth in claim 1 wherein the marginal portion has a radius of curvature which extends in the same direction as the radius of curvature of the central portion.
 3. A photocathode as set forth in claim 1 wherein the marginal portion has a radius of curvature which extends in the opposite direction as compared to the radius of curvature of the central portion.
 4. A photocathode as set forth in claim 2 wherein the effective electron emitting surface comprises a hyperbolic surface of revolution.
 5. An image intensifier tube comprising: an evacuated envelope having an input faceplate and an output faceplate; an aspherically curved photocathode disposed within the envelope adjacent the input faceplate; said photocathode including an effective electron emitting surface having a concave central portion and an encircling marginal portion adjacent its periphery; said marginal portion having a radius of curvature greater in magnitude than the radius of curvature of said central portion and the entire electron emitting surface including both of said portions being effective in the formation of an electron image in response to an incident radiational image; and a substantially flat imaging screen disposed within the envelope adjacent the output faceplate and axially aligned therewith.
 6. An image intensifier tube as set forth in claim 5 wherein a layer of fluorescent material is coaxially disposed between the input faceplate and the photocathode.
 7. An image intensifier tube as set forth in claim 5 wherein an anode sleeve is coaxially disposed between the photocathode and the imaging screen and is axially spaced from the photocathode.
 8. An image intensifier tube as set forth in claim 7 wherein a tubular focusing electrode is coaxially disposed between the photocathode and the anode sleeve and axially spaced therefrom.
 9. An image intensifier tube comprising: a tubular envelope having an input faceplate at oNe end and an output faceplate at the other end, the input faceplate having an aspherically curved inner surface including a concave central portion, a contiguous intermediate annular portion and a successively contiguous encircling marginal portion adjacent its periphery, the intermediate and marginal portions having respective radii of curvatures greater in magnitude than the radius of curvature of the central portion; a conforming layer of photoemissive material on the inner surface of the input faceplate, the entire photoemissive layer including the concave central portion, the intermediate annular portion and the encircling marginal portion adjacent its periphery being effective in the formation of an electron image in response to an incident radiational image; an anode sleeve disposed within the envelope in spaced coaxial relationship with the photoemissive layer; and a substantially flat imaging screen disposed within the envelope in spaced coaxial relationship with the anode sleeve and located adjacent said output faceplate. 